The present application relates to a lighting device having a blue light-emitting element and green phosphor layer, and more particularly to a lighting device which is used in a display device such as liquid crystal television set thanks to its high reliability even if an arbitrary green phosphor is used.
Recent years have seen a brisk development of backlights for liquid crystal television sets. Backlight schemes can be broadly classified into two types: (1) white LED (light-emitting diode) obtained by potting a blue LED with transparent resin having a yellow or green phosphor adapted to emit a blue light beam and red phosphor dispersed therein and (2) tricolor LED using blue, green and red LEDs.
The white LED classified as type (1) is characterized in that a simple driver system can be achieved because a backlight can be formed by driving the blue LED element adapted to emit a blue light beam. The tricolor LED classified as type (2) is characterized in that a liquid crystal television set having excellent color purity can be provided thanks to narrow emission spectral line widths from the LEDs emitting blue, green and red light beams. With type (1), however, the wavelength of light emission from the red phosphor has to be lengthened to achieve a wide color range. However, lengthening the light emission leads to significantly reduced brightness. On the other hand, type (2) is problematic due to its low efficiency of white light attributable to low efficiency of the green LED.
The following mention is made in Japanese Patent Laid-Open No. Hei 11-249604 (paragraph 0019, paragraphs 0033 to 0083, paragraphs 0091 to 0093, FIGS. 4 and 5) which is titled “Surface-Illuminated Display Device.”
The surface-illuminated display device performs display by illuminating a given display surface and includes a (a) light source having a first light-emitting substance adapted to emit a light beam of a first wavelength and a second light-emitting substance adapted to emit a light beam of a wavelength different from the first wavelength and (b) fluorescent plate having incident and emitting surfaces. The incident surface receives a light beam from the light source. The emitting surface faces the display surface. The fluorescent plate converts part of the light beam of the first wavelength into a light beam of the second wavelength longer than the first wavelength.
The following mention is made in Japanese Patent Laid-Open No. 2005-72479 (paragraphs 0021 to 0022, paragraphs 0056 to 0058, FIG. 9) which is titled “White Light-Emitting Device.”
The white light-emitting device may include two LEDs. One of the two LEDs includes, as a phosphor ZnSxSe1-x (0<x<1), at least one of phosphor ZnSxSe1-x (0.7≦x≦0.9) containing at least Au or Cu as an activator and phosphor ZnSxSe1-x (0.5≦x≦0.8) containing Ag as an activator, and emits a light beam having a wavelength in the range from 410 nm to 490 nm. Another LED emits a red light beam.
In this configuration, one of the phosphors takes charge of green (G), thus allowing for the blue and red LEDs to form an RGB white light-emitting element. Therefore, the document claims that it is possible to produce a white light beam which is applicable, without any problem, to all applications including red light beam.
The following mention is made in Japanese Patent Laid-Open No. 2007-273998 (paragraph 0014, paragraphs 0022 to 0023, FIG. 2) which is titled “White LED for Backlight Having a Phosphor Plate.”
The document claims that the white LED adapted to backlight the LCD (liquid crystal display) includes a blue LED added with preformed red and green phosphor platelets.
The following mention is made in Japanese Patent Laid-Open No. 2007-4099 (paragraphs 0014 to 0015, paragraphs 0055 to 0064, paragraph 0071, paragraphs 0078 to 0085, FIGS. 2 and 3) (hereinafter referred to as Patent Document 1) which is titled “Surface Light Source Device and Color Liquid Crystal Display Device Assembly.”
A surface light source device according to a first embodiment in Patent Document 1 is a surface light source device (more particularly a direct-type surface light source device) for lighting a transmissive color liquid crystal display device from the rear panel side. The liquid crystal display device includes a (a) front panel having a first transparent electrode, (b) rear panel having a second transparent electrode, (c) liquid crystal material provided between the front and rear panels. The surface light source device includes a (A) light source, (B) supporting member, (C) second primary color light-emitting layer and (D) third primary color light-emitting layer. The light source is arranged to be opposed to the rear panel and emits a first primary color light beam associated with a first primary color of the three primary colors of light which include first to third primary colors. The supporting member is provided between the rear panel and light source. The second primary color light-emitting layer is formed on the region of the supporting member associated with a subpixel adapted to emit a second primary color light beam corresponding to a second primary color of the color liquid crystal display device. The second primary color light-emitting layer includes second primary color light-emitting particles adapted to emit the second primary color light beam. The second primary color light-emitting layer is excited by energy radiation from the light source to emit the second primary color light beam. The third primary color light-emitting layer is formed on the region of the supporting member associated with a subpixel adapted to emit a third primary color light beam corresponding to a third primary color of the color liquid crystal display device. The third primary color light-emitting layer includes third primary color light-emitting particles adapted to emit the third primary color light beam. The third primary color light-emitting layer is excited by energy radiation from the light source to emit the third primary color light beam.
The following mention is made in Japanese Patent Laid-Open No. 2008-117879 (paragraph 0010, paragraphs 0032 to 0033, FIG. 5) (hereinafter referred to as Patent Document 2) which is titled “Planar Light-Emitting Device.”
This provides a planar light-emitting device. The planar light-emitting device includes a diffusion plate, first light-emitting elements and wavelength conversion sheet. The diffusion plate has main and rear surfaces, evens out the intensity distribution of the incident light beam entering from the rear surface and emits a light beam having a uniform intensity distribution. The first light-emitting elements are arranged on the side of the rear surface of the diffusion plate in a two-dimensional manner and each emit a blue light beam. The wavelength conversion sheet is provided on the side of the main or rear surface of the diffusion plate and includes a wavelength conversion material adapted to absorb the light beams from the first light-emitting elements and emit a green light beam.
FIG. 9 is FIG. 5 shown in Patent Document 2 and a schematic perspective view of a planar light-emitting device according to a third embodiment disclosed in Patent Document 2.
In this specific example, red and blue light-emitting elements 102 and 103 are arranged in a two-dimensional manner on a substrate 101 on the side of a rear surface of a diffusion plate 110 having main and rear surfaces. Either a space or a ight-transmitting material such as resin may be provided between the light-emitting elements 102 and 103 and diffusion plate 110.
Also in this case, the red and blue light-emitting elements 102 and 103 can be provided on the substrate roughly at a ratio of 1:2. In the present embodiment, a low efficiency green light-emitting element is also omitted. Instead, a wavelength conversion sheet 130 is provided on the surface of the diffusion plate 110. The wavelength conversion sheet 130 includes a green wavelength conversion material which matches the LCD color filter. Patent Document 2 claims that the third embodiment provides reduced overall cost and prevents efficiency deterioration thanks to the omission of the low efficiency green LED as with the first and second embodiments.
The following mention is made in JP-T-2003-529889 (paragraph 0012, paragraphs 0021 to 0022, FIG. 1) (hereinafter referred to as Patent Document 3) which is titled “Lighting Device.”
An extremely excellent embodiment of a lighting device according to a first concept disclosed in Patent Document 3 is characterized in that at least two light-emitting diodes include a blue light-emitting diode and at least one red light-emitting diode and that the conversion means include a luminescent material adapted to convert part of the light beam from the blue light-emitting diode into a green light beam.
It should be noted that descriptions about a phosphor sheet are given, for example, in Japanese Patent Laid-Open No. 2004-161808 (paragraphs 0067 to 0068, FIG. 1), Japanese Patent Laid-Open No. 2005-77448 (paragraph 0068) (hereinafter referred to as Patent Document 4), Japanese Patent Laid-Open No. 2006-126109 (paragraph 0157), Japanese Patent Laid-Open No. 2007-85797 (paragraphs 0065 to 0070, paragraphs 0077 to 0078), and Japanese Patent Laid-Open No. 2008-50593 (paragraphs 0006 to 0010, paragraph 0066, paragraphs 0085 to 0086, FIG. 3)